1. Field of the Invention
The present invention relates to display systems in general and more particularly to electronic price tags which are used on the shelves of a supermarket or the like for displaying information regarding shelved items.
2. Prior Art
The use of electronic price tags for displaying prices and other product information is beginning to gain acceptance in the retail industry. In most retail establishments such as drug stores, supermarkets, etc., items for sale are displayed along a shelf. Information about the various items are provided by the electronic tags which are placed along the shelves holding the items. Changes in item names and/or prices can be done electronically from a computer which is in the store or at a remote site, such as a company's headquarters.
The electronic price tags are attractive to store operators because they ensure accurate price markings at the shelf and save labor costs in making price changes. In other words, the electronic price tags are attractive because of labor savings relating to price changes and improved accuracy over conventional printed label tags.
U.S. Pat. Nos. 4,821,291 and 4,654,514 describe electronic pricing systems in which the electronic shelf labels are fabricated from segmented LCD technology. Electrical power for the display modules is provided by power conductors positioned along the shelves, a battery, or by solar cells placed on the casing of the module.
Even though the segmented LCD technology is inexpensive and as such is the preferred technology for fabricating shelf label modules, it suffers from the problem of being susceptible to display errors. The susceptibility stems from the segments that form the character to be displayed. As is shown in the '514 patent, an alphanumerical character is usually formed from seven segments. If a segment or its connection is damaged, a wrong number could be displayed. For example, an "8" could be displayed as a "6," "0," or a "9." As a result, the reliability of electronic shelf label modules is being questioned. In addition, some governments impose severe penalties for shelf pricing errors. To overcome these problems, there is a need to include error detection and correction mechanisms on the electronic shelf label modules.
U.S. Pat. No. 4,758,069 describes a device and method for determining if the drive circuit of an LCD fails. Each display segment is driven alternately by two circuits. If the circuits alternate at a relatively slow rate and an error occurs in one of the driving circuits, then each segment of the display is driven at a 50% duty cycle and flickers or blinks. The flickering indicates a malfunctioning in the drive circuits. The teachings of this patent only detect failure in drive circuits. However, failures in the display connections and/or in the segments per se are not detected.
U.S. Pat. No. 4,734,688 describes an LCD system in which malfunction in the drive circuit causes one or more segments of the display to flicker. Each segment of the display is driven by a drive circuit and a test circuit. The drive circuit generates a strong drive signal which is applied to a display segment. The test circuit generates a weak checking signal which is superimposed on the strong signal. The weak signal has no discernible effect on the display segment so long as the strong signal is present. However, any defect in the drive circuit or between the drive circuit and the contact to the display segment suppresses the strong signal and allows the weak signal to be discerned in the form of a flicker of the relevant segment electrode. The patent does not detect failures in the display segments.
U.S. Pat. No. 4,311,993 describes an LCD system having circuits which use the capacitive properties of the segments to indicate malfunctioning of the segments.
U.S. Pat. No. 4,095,217 describes an LCD for a calculator.
The display includes an upper glass plate and a lower glass plate which are spaced from each other by a liquid seal. A nematic liquid is poured in the cavity which is formed between the glass plates. Upper and lower sets of transparent electrodes are placed on the surfaces, interfacing the liquid, of the upper and lower glass plates. Power for the calculator is provided by a photovoltaic converter which is positioned on the bottom surface of the lower glass plate to capture light which is incident through the display.
It should be noted that U.S. Pat. No. 4,095,217 sets forth the structure for a conventional LCD. In addition, the patent does not disclose a fail safe feature. Even though the structure appears to work well for its intended purpose, there may be applications for an LCD which does not use a nematic liquid or a LCD which uses a nematic liquid and a fail safe feature. Such a LCD, including fail safe features, is described hereinafter.